Method for air conditioning a motor vehicle

ABSTRACT

A method for air conditioning a motor vehicle, in which, in a heating mode, the passenger compartment of the motor vehicle is heated via a passenger compartment heat exchanger ( 5 ) through heat pump operation of a refrigerating circuit ( 1 ). To prevent fogging of the windows of the motor vehicle from the outset, the temperature and humidity in the passenger compartment are recorded by measurement technology, and when the temperature is in a predefined range and the atmospheric humidity reaches a defined threshold, the mass flow of refrigerant in the circuit is throttled upstream of the passenger compartment heat exchanger ( 5 ), in such a manner that the moisture contained in the air stream passing the passenger compartment heat exchanger ( 5 ) is at least substantially condensed at the passenger compartment heat exchanger ( 5 ), and the moisture which has already condensed at the heat exchanger ( 5 ) remains at the heat exchanger ( 5 ). The passenger compartment is heated by a heat source which is outside the circuit ( 1 ) until the temperature in the passenger compartment exceeds an upper limit temperature of the predefined range.

This application is a national stage of PCT/EP2003/013630 filed Dec. 3,2003 and based upon DE 102 61 179.3 filed Dec. 20, 2002 under theInternational Convention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method for air conditioning a motor vehicle.

2. Related Art of the Invention

A method of the generic type is known from DE 36 35 353 A1, in which itis described that the air-conditioning system should be run in heat pumpmode in order to rapidly heat a passenger compartment and to defrostfrozen or fogged windows. In this case, the actual condenser of theair-conditioning circuit is bypassed downstream of the compressor via abypass line, after which the heated cooling medium passes through theevaporator, which functions as a condenser and also has an air streampassed into the passenger compartment passing through it. The coolingmedium which has cooled there then takes up heat from a third heatexchanger, which serves as an evaporator and has the coolant of theinternal combustion engine flowing through it. During a cold start ofthe engine and at low ambient temperatures of between −10 and +10° C.,moisture has usually condensed or frozen on the evaporator. However, theair flowing past the evaporator into the passenger compartment takes upsome of the moisture which is present at the evaporator and transportsit, inter alia, onto the windows of the motor vehicle, which for acertain period of time leads to undesirable fogging of the windows.

SUMMARY OF THE INVENTION

The invention is based on the object of developing a method of thegeneric type in such a manner that fogging of the windows of the motorvehicle is prevented from the outset.

According to the invention, this object is achieved by the features setforth below.

As a result of the humidity within the passenger compartment in relationto the passenger compartment temperature being recorded by measurementtechnology and of a heat source which is external to the refrigerationcircuit being used, the heat source for heating the passengercompartment is started up when a threshold value for the humidity at adefined temperature is reached. At the same time, the mass flow ofrefrigerant through the evaporator is throttled to a sufficient extentfor there to be virtually no introduction of heat into the passengercompartment via the evaporator. As a result, the condensate remains atthe heat exchanger forming the evaporator, or the feed air to thepassenger compartment is dried by condensation at the evaporator. Theexternal heat source as it were functionally replaces the evaporator forheating the passenger compartment. Only when a defined temperature levelhas been reached in the passenger compartment is the external heatsource decoupled and the throttling of the mass flow of refrigerantthrough the evaporator relieved. This creates air-conditioning in whichthe passenger compartment can be heated at low temperatures withoutfogging of the windows. The method according to the invention isparticularly advantageous when CO₂ is used as refrigerant, on account ofthe superior use qualities of CO₂ compared to other refrigerants in heatpump mode, which derive from its physical properties at the givenpressure level.

BRIEF DESCRIPTION OF THE INVENTION

Expedient configurations of the invention are to be found in thesubclaims; moreover, the invention is explained in more detail below onthe basis of an exemplary embodiment illustrated in the drawings, inwhich the FIGURE diagrammatically depicts an air-conditioning circuit ofa method according to the invention with a heating heat exchanger asexternal heat source.

DETAILED DESCRIPTION OF THE INVENTION

The FIGURE illustrates an air-conditioning circuit 1 of anair-conditioning system of a motor vehicle which can be used both tocool a passenger compartment of the motor vehicle and to heat thepassenger compartment. The basic components of the circuit comprise acompressor 2, a condenser 3, a throttle valve 4 and an evaporator 5 usedas passenger compartment heat exchanger. In heating mode, the circuit 1is switched to heat pump operation, according to which the hotrefrigerant—CO₂—which has been compressed to a high pressure level bythe compressor 2 is made to bypass the condenser 3 via a bypass line 6.The hot refrigerant then passes a branch line 7, which is connected tothe bypass line 6 and has a 3/2-way valve 8, from which it flows throughthe heat flux section of a countercurrent heat exchanger 9 of acontinuation line 10, releasing a small amount of its heat to the coolcountercurrent. Furthermore, the refrigerant, which is still relativelyhot, flows through the throttle valve 4 and the downstream evaporator 5,which has feed air for the passenger compartment passing through it. Inthe process, the refrigerant releases at least the majority of its heatto the air stream which is used to heat the passenger compartment. Therefrigerant, which is now cold, then flows onward via a 2/2-way valve toa collection tank 11, in which liquefied refrigerant fractions arestored in order to avoid damage to the compressor 2 when the refrigerantis sucked in and to serve as a reservoir when the demand for refrigerantrises. The collection tank 11 is followed by the cooling flux section ofthe countercurrent heat exchanger 9, in which the cold refrigerant isheated again to some extent, so that it can be entirely in gas form whenit is sucked in by the subsequent compressor 2.

In addition to the air-conditioning circuit 1 which has been outlined,the vehicle is also equipped with an engine cooling circuit 12. Thiscircuit comprises two sections 13 and 14, which are fluidically coupledto one another via a 4/2-way valve 15. Whereas an engine 16, a radiator17, an exhaust-gas heat exchanger 18, a thermostatic valve 19 and a heatexchange section of a countercurrent heat exchanger 20, with theopposite part of the section incorporated in a secondary line 21, whichbranches off downstream of the evaporator 5, of the circuit 1, arearranged in section 13, the section 14 is equipped with a heating pump22, a heat exchange section of a countercurrent heat exchanger 23, whichsection is thermally coupled to its opposite part arranged in the bypassline 6, and a heating heat exchanger 24. The heat exchanger 24, togetherwith the evaporator 5, is arranged in an air-conditioning box 25 of theair-conditioning system.

Furthermore, the temperature and, in addition, the atmospheric humidityin the passenger compartment are recorded by measurement technologyusing suitable sensors, with the sensors which detect the humiditypreferably being assigned to one or more windows. To fundamentallyprevent fogging of the windows at low temperatures, first of all thecircuit 1 is switched to heat pump operation, after which the hotrefrigerant is passed through the compressor 2, via the bypass line 6,the branch line 7 and the throttle valve 4, to the evaporator 5, whichfirst of all heats the passenger compartment by means of the air streamwhich passes through it and to which it releases a large proportion ofheat. At the same time, however, the refrigerant is already transferringheat to the coolant of the circuit 12 at the countercurrent heatexchanger 23 of the circuit 12. In this phase, the circuit 12 isconnected in such a way by the 4/2-way valve 15 that its two sections 13and 14 are isolated from one another fluidically and completely withregard to heat transfer. Therefore, the heating pump 22 merelycirculates the coolant of circuit 12 through section 14.

If the temperature is in a predefined temperature range and theatmospheric humidity reaches a defined threshold, after whichunacceptable fogging of the windows will occur, the throttle valve 4 iscontrolled by signals from the temperature and humidity sensors in sucha way that the mass flow of refrigerant in the circuit 1 upstream of thepassenger compartment heat exchanger 5 is throttled to only a lowthroughput. In the process, the moisture contained in the air streampassing through the passenger compartment heat exchanger 5 is at leastsubstantially condensed at the passenger compartment heat exchanger 5,while the moisture which has already condensed at the heat exchanger 5remains at the heat exchanger 5. The coolant which is circulating insection 14 of the cooling circuit 12 has in the meantime been heated bythe introduction of heat from the circuit 1 via the countercurrent heatexchanger 23 following the first heating phase, with the heat exchanger24 now performing the initial role of the evaporator 5 and heating thepassenger compartment by means of an air stream passing through it. Inthis case, the heat exchanger 24 compensates for the loss of heatingpower which results from the throttling of the mass flow of refrigerantupstream of the evaporator 5, and at the same time the evaporator 5dries the air which is passed to the passenger compartment.

The heating of the passenger compartment by means of the heat exchanger24 serving as heat source continues until the temperature in thepassenger compartment exceeds an upper limit temperature of thepredefined temperature range, above which it is impossible for anyfogging precipitation to form on the windows, even at relatively highhumidity levels. Then, the throttle valve 4 is opened again by means ofthe abovementioned sensors, so that the mass flow of refrigerant in thecircuit 1 increases such that the passenger compartment can besufficiently heated by way of the evaporator 5. The function of theheating heat exchanger 24 becomes subordinate if not completelyirrelevant, and air no longer passes through it.

The heat which continues to be introduced into the circuit can now beused entirely to heat the engine 16 and the radiator 17, with the4/2-way valve 15 being connected accordingly and the two circuitsections 13 and 14 being fluidically connected to one another.

Although it is conceivable for the passenger compartment to be heated bya different heat source located outside the circuit 1, it is highlyadvantageous to use the heating heat exchanger 24 of the engine coolingcircuit 12, which is already present in any case, as the heat source,with a view to saving space.

Furthermore, it is also possible for the circuit 12 to be providedwithout a 4/2-way valve 15, which eliminates the need to disconnect thetwo circuit sections 13 and 14 from one another. Although thisstructurally simplifies the engine cooling circuit 12, the heat whichhas been introduced into this circuit 12 from the circuit 1 isdistributed not just through section 14 but through the entire circuit12, which leads to significantly slower and, in the worst possiblescenario, inadequate heating of the passenger compartment. It is alsoconceivable to dispense with the countercurrent heat exchanger 23, whichagain represents a structural simplification. In this case, the onlycoupling between the two circuits 1 and 12 is via the countercurrentheat exchanger 20 which, however, has no effect on the heating of thepassenger compartment. In this case, however, there is no heating of thepassenger compartment when the engine is cold. Rapid heating of theengine is achieved mainly with relatively small engine designs, andconsequently even halfway comfortable heating of the passengercompartment can only be expected with these designs of engine. Bycontrast, if the countercurrent heat exchanger 23 is provided, thecompressor power can easily be used to heat the passenger compartmentquickly and comfortably.

Furthermore, the heating of the passenger compartment is improved interms of efficiency and speed if the air-conditioning system is switchedto recirculated air and consequently the supply of fresh air is stopped.In this case, the refrigerant releases its heat in the countercurrentheat exchanger 23 and is throttled in throttle valve 4 to a pressurewhich correlates with a temperature such that the temperature at thesurface of the passenger compartment heat exchanger 5 is below thedewpoint temperature which leads to fogging of the windows.

1. A method for air conditioning a passenger compartment of a vehicle, said method comprising: providing heat from a passenger compartment heat exchanger to the passenger compartment through heat pump operation of a refrigerating circuit (1) comprising a compressor (2) for pressurizing an automobile refrigerant, a condenser (3), a throttle valve (4) and an evaporator (5) used as a passenger compartment heat exchanger; measuring the temperature in the passenger compartment with a temperature sensor; measuring the atmospheric humidity in the passenger compartment with a humidity sensor; and when the temperature is in a predefined range and the atmospheric humidity reaches a defined threshold: controlling throttle valve (4) by signals from the temperature and humidity sensors to throttle the mass flow of refrigerant in the circuit upstream of the passenger compartment heat exchanger (5), in such a manner that the moisture contained in the air stream passing the passenger compartment heat exchanger (5) is at least substantially condensed at the passenger compartment heat exchanger (5), and the moisture which has already condensed at the heat exchanger (5) remains at the heat exchanger (5), and heating the passenger compartment by a heat source which is outside the circuit (1) until the temperature in the passenger compartment exceeds an upper limit temperature of the predefined range.
 2. The method as claimed in claim 1, wherein said temperature range and atmospheric humidity threshold are determined based on the dewpoint temperature which leads to fogging of the windows.
 3. The method as claimed in claim 1, wherein said a heat source which is outside the circuit (1) includes a heating heat exchanger (24) which has heated engine coolant from an engine cooling circuit (12) flowing through it, and also has an air stream that is routed into the passenger compartment passing through it.
 4. The method as claimed in claim 3, comprising heating, prior to throttling the mass flow of refrigerant in the refrigerating circuit (1), both the refrigerant and the engine coolant by means of the compressor (2), which acts in the heat pump, of the refrigerating circuit (1), with heat being removed from the refrigerating circuit (1) via a countercurrent heat exchanger (23) and transferred to the engine coolant.
 5. The method as claimed in claim 4, wherein the heat is transferred only to the engine coolant of a first section (14) of the engine cooling circuit (12), which first section (14) includes a heating pump (22) and the heating heat exchanger (24), and wherein a second circuit section (13) of the engine cooling circuit (12), which includes the engine (16) and the radiator (17), are fluidically decoupled from the first circuit section (14).
 6. The method as claimed in claim 1, wherein during heating, the air-conditioning system is switched to recirculated air, the refrigerant releasing its heat in the counter-current heat exchanger (23) and being throttled in the throttle valve (4) to a pressure which correlates with a temperature such that the temperature at the surface of the passenger compartment heat exchanger (5) is below the dewpoint temperature which leads to fogging of the windows. 